Liquid crystal display device and driving method thereof

ABSTRACT

A liquid crystal display device and a driving method for a liquid crystal display. The liquid crystal display device is formed to have thin film transistor conductive channels having an inverted “U”-shape. A shielding plate is provided to block incident light from reaching the conductive channels of the thin film transistors. This reduces or eliminates photo-induced leakage current and picture quality degradation. To prevent pixel electrode potential fluctuations caused by subsequent scan line drive signals, the liquid crystal display device is driven by scan signals that are sequentially applied from the mth gate line up to the first gate line.

[0001] This application claims the benefit of Korean Patent ApplicationNo. 81968/2001, filed on Dec. 20, 2001, which is hereby incorporated byreference for all purposes as if fully set forth herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] This invention relates to liquid crystal displays. Moreparticularly, this invention relates liquid crystal displays havingreduced photo leakage current and improved picture quality.

[0004] 2. Discussion of the Related Art

[0005] Cathode ray tubes (CRT) are widely used display devices intelevision sets, measurement instrumentation, and information terminals.However, because they are heavy and consume significant power, CRTs arenot well suited to applications that require compact, lightweight, lowpower displays.

[0006] A substitute for CRTs, liquid crystal displays, is compact,lightweight, and consumes low power. A liquid crystal display deviceincorporates a matrix of liquid crystal cells that are sequentiallyselected, line-by-line, to produce picture information. To do so, liquidcrystal cells vary their light transmittance in accord with data signalsthat carry picture information.

[0007] A liquid crystal display includes a liquid crystal panel havingliquid crystal cells and driver integrated circuits (IC) for drivingthose liquid crystal cells. A liquid crystal panel is usually comprisedof a thin film transistor array substrate and of a color filtersubstrate that are disposed in a facing relationship. Additionally, aliquid crystal layer is interposed between the thin film transistorarray substrate and the color filter substrate.

[0008] A thin film transistor array substrate includes a plurality ofdata lines for transmitting data signals from data driver integratedcircuits to the liquid crystal cells, and a plurality of gate lines fortransmitting scan signals supplied from gate driver integrated circuitsto the liquid crystal cells. The liquid crystal cells are defined byintersections of the data lines and the gate lines. As a gate driverintegrated circuit sequentially supplies scan signals to the gate lines,data signals are supplied to the liquid crystal cells.

[0009] A common electrode on the color filter substrate and pixelelectrodes on the thin film transistor array substrate are used toproduce electric fields across the liquid crystal layer. By controllingthe voltage applied to the pixel electrodes the light transmittance ofeach liquid crystal cell can be controlled.

[0010] To control the voltage applied to a pixel electrode, a thin filmtransistor is formed in each liquid crystal cell. When a scan signal issupplied to a gate electrode of the thin film transistor, a conductivechannel is formed between a source electrode, which is connected to adata line, and a drain electrode, which is connected to an associatedpixel electrode. Thus, the thin film transistor controls the flow ofdata signals to the pixel electrodes. Thin film transistors usually useamorphous silicon, which can be formed at a low temperature on alarge-scale insulation substrate, such as a low-priced glass substrate,as an active layer. Accordingly, controlling data signals applied toeach liquid crystal cell by selective switching of the thin filmtransistors, the light transmittance of the liquid crystal cells can becontrolled.

[0011] The light transmitting process of a liquid crystal display devicewill now be described. First, a common electrode voltage is supplied tothe common electrode. Then, scan signals are sequentially supplied tothe gate lines by gate driver integrated circuits. The scan signals areapplied to the gate electrodes of the thin film transistors. Meanwhile,data signals are supplied to the liquid crystal cells by data driverintegrated circuits via data lines. The data signals are applied to thesource electrodes of the thin film transistor.

[0012] Accordingly, the data signals are supplied to the drainelectrodes through conductive channels formed when a scan signal isapplied to a particular transistor. The data signal is supplied to thedrain electrode through the channel. The data signal is thus supplied tothe pixel electrode that is connected to the drain electrode. Inpractice, the pixel electrode is also connected to a storage electrode.Thus, the data signal voltage supplied to each pixel electrode is storedin a storage electrode. When a thin film transistor is turned off, thevoltage across its storage capacitor continues to be applied to thepixel electrode, thereby maintaining the liquid crystal cell drive.

[0013] As mentioned, since a common electrode voltage is applied to thecommon electrode, and a data signal voltage is applied to the pixelelectrode, electric fields are produced across the liquid crystal layerby the potentials of the common electrode and of the pixel electrodes.

[0014] When an electric field is applied across the liquid crystallayer, the liquid crystal is rotated by dielectric anisotropy toselectively transmit light emitted from a back light unit through thepixel electrode. The electric field strength is controlled by the datasignal voltage that is applied to the pixel electrode, and the lighttransmittance of the liquid crystal layer is controlled by the electricfield strength.

[0015] Unfortunately, continuous applying a single electric fieldpolarity degrades the liquid crystal. To prevent such degradation thedata signals alternately switch polarity relative to the common voltage.This general technique is called inversion driving.

[0016]FIG. 1 is an exemplary view showing voltage waveforms applied to aliquid crystal display device. As shown, a common electrode voltage(Vcom) is applied to the common electrode, while a data signal voltage(V_(DATA)) is applied to the source electrode of a thin film transistorvia the data line. Additionally, a scan signal (V_(G)) is applied to thegate electrode of the thin film transistor via the gate line.

[0017] During the turn-on period of the thin film transistor, when thescan signal (V_(G)) is applied at a high potential, the positive datasignal voltage (V_(DATA)) is supplied to the pixel electrode and to thesource capacitor by the drain electrode. At that time, the positive datasignal voltage (V_(DATA)) is charged into the storage capacitance. Thus,as shown, a pixel electrode voltage (V_(P)) is produced.

[0018] When the thin film transistor is turned off by removal of thescan signal (V_(G)), a voltage drop from the charged pixel electrodevoltage (V_(P)) occurs because of a parasitic capacitance. The voltagedrop is called a kick-back voltage (“ΔV_(P)”), reference FIG. 1.

[0019] During the turn-off period the pixel electrode voltage (V_(P))charged into the storage capacitor is applied to the pixel electrode,thus maintaining drive to the liquid crystal cell.

[0020] Meanwhile, in the n+1th frame, since the above-describedinversion driving method is used, a negative data signal voltage(V_(DATA)) is supplied through the source and drain electrodes to thepixel electrode and to the storage capacitor. Accordingly, as shown inFIG. 1, the pixel electrode voltage (V_(P)) in the n+1th frame has avoltage waveform that is symmetrical relative to the common electrodevoltage (Vcom) with the pixel electrode voltage (V_(P)) of the nthframe,

[0021] Meanwhile, since the thin film transistor channel is amorphoussilicon, if an external light is irradiated onto the channel aphoto-induced leakage current results. The photo-induced leakage currentdecreases the voltage of the storage capacitor during the turnoffperiod, which reduces the pixel electrode voltage (V_(P)) as shown inFIG. 1.

[0022] Since a transmission type liquid crystal display device does notemit light, it requires an optical source such as a back light unit orexternal light. A liquid crystal display device that uses a back lightunit is called a transmission type liquid crystal display device, whilea liquid crystal display device that uses external natural light iscalled a reflective type liquid crystal display device.

[0023] The transmission type liquid crystal display device usuallylocates the back light unit either below the liquid crystal displaypanel or along an edge. Currently, the edge-type transmission type ismore common.

[0024] However, the transmission type liquid crystal display device isinefficient in that only 3% to 8% of the light from the back light unitis actually transmitted. For example, using the reasonable assumptionsthat the transmittance of two polarization plates is about 45%, that thetransmittance of two glass substrates is about 94%, that thetransmittance of a thin film transistor array and pixel is about 65%,and that the transmittance of a color filter is about 27%, then theoverall transmittance of a liquid crystal display device is about 7.4%.

[0025] Thus, the amount of light from a transmission type liquid crystaldisplay device is only about 7% of the light from the back light unit.Thus, if a high luminance is required the back light unit needs to bevery bright, something that causes great power consumption. Thus, inorder to supply enough power to the back light unit a large, heavy, highcapacity battery is required. Even with such a battery there is a limiton how long the liquid crystal display device can be used whiletraveling. Further, such a large capacity battery is an obstacle toachieving the desired size, weight, and portability.

[0026] A solution to the power problems of the transmission type liquidcrystal display device is the reflective type liquid crystal displaydevice. The reflective type liquid crystal display produces an imageusing external light. Thus, only a small amount of power is required.Accordingly, a reflective type liquid crystal display device can be usedfor extended periods of time, is more compact, lightweight and portable.Furthermore, since the entire unit pixel can be used, the aperture ratioof a reflective type liquid crystal display device is excellent.

[0027] The reflective type liquid crystal display device includes atranslucent reflective electrode that is made of a light reflectivemetal, instead of the transparent conductive material used in atransmission type liquid crystal display device. The reflectiveelectrode produces an electric field across the liquid crystal layer inconjunction with a common transparent electrode on the color filtersubstrate.

[0028] When an electric field is applied across the liquid crystallayer, the liquid crystal is rotated by the dielectric anisotropy. Thiscontrols the amount of external light that is transmitted through thecolor filter substrate, and thus the amount of light reflected by thereflective electrode. The reflected light is thus controlled by voltagesapplied to the reflective electrodes.

[0029] However, the reflective type liquid crystal display device has aproblem in that since the materials of the reflective electrode and thecommon transparent electrode are different, the driving characteristicsof the liquid crystal are deteriorated, which results in degradation ofthe image produced on the liquid crystal display device.

[0030] In addition, the external light required for the reflective typeliquid crystal display device is not constant. That is, while areflective liquid crystal display device can be used during the day orwhen artificial light exists, it will not work in the dark.

[0031] Consequently, transmission/reflective type liquid crystal displaydevices have been proposed. The transmission/reflective type liquidcrystal display device adopts a reflection mode when external light isavailable, but a transmission mode when external light is not available.

[0032] The transmission/reflective type liquid crystal display devicewill be described with reference to the accompanying drawings. FIG. 2shows a plan view of a unit pixel of a transmission/reflective typeliquid crystal display device. With reference to FIG. 2, gate lines 104are arranged at regular intervals on a substrate, and data lines 102 arearranged at regular intervals, but in a crossing relationship.Accordingly, the gate lines 104 and the data lines 102 form a matrix ofliquid crystal cells. A thin film transistor (TFT), a reflectiveelectrode 114 and a pixel electrode 115 are provided in each liquidcrystal cell.

[0033] Each thin film transistor includes a gate electrode 110 thatextends from a gate line 104, and a source electrode 108 that extendsfrom a data line 102 and that overlaps the gate electrode 110.Additionally, each thin film transistor includes a drain electrode 112that corresponds to the source electrode 108 on the gate electrode 110.Each thin film transistor (TFT) also includes an active layer (not shownin FIG. 2) for forming a conductive channel between the source electrode108 and the drain electrode 112 when a scan signal is supplied to thegate electrode 110. As the active layer, amorphous silicon is beneficialin that it can be formed at a low temperature on a low-priced glasssubstrate.

[0034] Extending the conductive channel tends to improve thecharacteristics of the thin film transistor (TFT). Thus, the conductivechannel is preferably formed in an “L” shape or in a “U” shape. FIG. 2illustrates the “U” shape. To achieve a “U”-shaped conductive channel,the source electrode 108 extends with a hook shape from the data line102, and the drain electrode 112 is inside the hook.

[0035] Compared to an “L”-shaped conductive channel, the “U”-shapedconductive channel is longer. Furthermore, the overlap between the drainelectrode 112 and the gate electrode 110 can be formed despite somemisalignment in the fabrication process. However, the overlap betweenthe drain electrode 112 and the gate electrode 110 is significantlyinfluenced by misalignment. Thus, the parasitic capacitance (Cgd)between the drain electrode 112 and the gate electrode 110 can bechanged enough that picture quality degradation results.

[0036] The pixel electrode 115 and the drain electrode 112 electricallyconnect through a drain contact hole 116 that is formed through aninsulation film (not shown in FIG. 2). The pixel electrode 115 is formedin the pixel region of each liquid crystal cell and is comprised of atransparent conductive material.

[0037] At marginal portions of the each pixel region is a translucentreflective electrode 114 that is comprised of a highly reflective andconductive material. The reflective electrode 114 is overlapped by theinsulation film and by the pixel electrode 115. The reflective electrode114 is also formed near the thin film transistor (TFT). Thus, thereflective electrode acts as a shielding plate 114A that blocks lightthat is directed toward the conductive channel of the thin filmtransistor (TFT). Because of the shielding plate 114A, light irradiatedtoward the conductive channel is blocked, and thus photo-induced currentleakage of the thin film transistor (TFT) is reduced or prevented.

[0038] Referring once again to FIG. 1, by minimizing the reduction ofthe pixel electrode voltage (V_(P)) charged into the storage capacitorduring the turn-off period of the thin film transistor (TFT), thepicture quality of the liquid crystal display device could be improved.

[0039] In conventional transmission/reflective type liquid crystaldisplays of the type used in notebook personal computers, if theexternal illumination is below about 50,000 to 60,000 (Lux), that lightis blocked by the shielding plate 114A. Referring now to FIG. 3, but ifthe external illumination is around 100,000˜110,000 (Lux), somethinglike noon sun light, incident light having an incident angle of lessthan 30° relative to the display unit of the notebook type personalcomputer is not blocked. Problems can arise because the shielding plate114A does not sufficiently block the light.

[0040] Referring once again to FIG. 2, if the shielding plate 114A failsto sufficiently block the light incident at regions “A” to “D”photo-induced leakage currents and their consequent picture qualitydegradation can result.

[0041]FIG. 4 is a sectional view of the thin film transistor taken alongline I-I′ of FIG. 2. As shown, the sectional structure includes the gateelectrode 110 on a substrate 101, and a gate insulation film 130 overthe substrate 101 and over the gate electrode 110. Beneficially, thegate electrode 110 is formed along with the gate line 104.

[0042] Still referring to FIG. 4, an active layer 136 is on the gateinsulation film 130 and over the gate electrode 110. The active layer136 includes an amorphous silicon semiconductor layer 132 and an n+amorphous silicon ohmic contact layer 134 that is highly doped withphosphor. The drain electrode 112 is located above the center of theactive layer 136, while the source electrode 108, which is also abovethe active layer 136, is located away from the drain electrode 112toward the edges of the active layer 136.

[0043] The ohmic contact layer 134 is partially removed duringpatterning of the source electrode 108 and the drain electrode 112 toassist defining the “U” shaped channel.

[0044] Still referring to FIG. 4, a passivation film 138 is formed overthe source electrode 108 and the drain electrode 112, over the activelayer 136, and over the gate insulation film 130. The passivation film138 can be an inorganic insulation film, such as SiNx or SiOx, or toimprove the aperture ratio, the passivation film 138 can be an organicinsulation film such as benzocyclobutane (BSB), spin-on-glass (SOG) oracryl having a low dielectric constant. If the reflective electrode 114(described in more detail subsequently) is directly deposited on anorganic insulation film, to prevent contamination of the depositionchamber by organic materials, the passivation film 138 can be formed bystacking organic and insulation films.

[0045] Still referring to FIG. 4, the reflective electrode 114 and theshielding plate 114A are on the passivation film 138. Those structurescan be simultaneously patterned. Then, an inorganic insulation film 140,such as SiNx or SiOx, is formed over the passivation film 138 and overthe reflective electrode 114. The inorganic insulation film 140electrically insulates the reflective electrode 114 and a pixelelectrode 124 (described below) such that when an electric field isapplied between the common electrode (not shown) on the transparentcolor filter substrate and the translucent reflective electrode 114,deterioration of the liquid crystal due to different materials isprevented.

[0046] Still referring to FIG. 4, a drain contact hole 116 is thenformed through the passivation film 138 and through the inorganicinsulation film 140 so as to expose a portion of the drain electrode112. Then, the pixel electrode 124 is formed over the inorganicinsulation film 140 and into the drain contact hole 116 so as to contactthe drain electrode 112.

[0047] Referring now back to FIG. 2, the gate lines 104 extendperpendicular to the gate electrodes 110. The gate lines 104 act as afirst electrode of a storage capacitor. An insulation film overlaps thefirst electrode, and then a storage electrode (not shown) overlaps thefirst electrode with the insulation film acting as a dielectric layer,thus forming a storage capacitor 118. The storage electrode 118 connectsto a pixel electrode 115 through a storage contact hole 122.

[0048]FIG. 5 is a cross-sectional view of the storage capacitor 118taken along line II-II′ of FIG. 2. As shown, the storage electrodeincludes the first electrode 119 on the substrate 101. The gateinsulation film 130 covers the substrate 101 and the first electrode119. The storage electrode 120 is formed on the gate insulation film 130and over part (reference FIG. 2) of the first electrode 119.

[0049] Still referring to FIG. 5, the passivation film 138 and theinorganic insulation film 140 are stacked over the gate insulation film130 and over the storage electrode 120. The reflective electrode 114 isformed as previously described. A storage contact hole 122 is formedthrough the inorganic insulation film 140 and through the passivationfilm 138 to expose a portion of the storage electrode 120. A pixelelectrode 124 is then formed over the inorganic insulation film 140,into the storage contact hole 122, and in electrical contact with thestorage electrode 120.

[0050] Accordingly, the storage electrode 120 overlaps the firstelectrode 119 with an interposed gate insulation film 130, therebyforming the storage capacitor 118.

[0051] The storage capacitor 118 is charged to a data signal voltageduring the turn-on period of the thin film transistor (TFT) (when a scansignal is applied to the gate line 104). The charged voltage is thenapplied to the pixel electrode 124 during the turn-off period of thethin film transistor (TFT), thereby maintaining the state of the liquidcrystal in the OFF period.

[0052] As mentioned, a conventional liquid crystal display device in anotebook type personal computer can have a degraded picture when lightis incident at an angle of less than 30°. This is because the thin filmtransistor conductive channel receives such incident light, whichcreates photo-induced leakage current, which causes picture qualitydegradation.

[0053] Therefore, a new liquid crystal display that does not suffer fromphoto-induced leakage current when light is incident at an angle of lessthan 30° would be beneficial.

SUMMARY OF THE INVENTION

[0054] Accordingly, the present invention is directed to a liquidcrystal display device and driving method thereof that substantiallyobviates one or more of the problems due to limitations anddisadvantages of the related art.

[0055] An advantage of the present invention is to provide a liquidcrystal display device and its driving method that improves picturequality by minimizing photo-induced leakage current.

[0056] Additional features and advantages of the invention will be setforth in the description which follows, and in part will be apparentfrom the description, or may be learned by practice of the invention.The objectives and other advantages of the invention will be realizedand attained by the structure particularly pointed out in the writtendescription and claims hereof as well as the appended drawings.

[0057] To achieve these and other advantages and in accordance with thepurpose of the present invention, as embodied and broadly described,there is provided a liquid crystal display device having a plurality ofgate lines and a plurality of data lines that cross the gate lines. Gateelectrodes extend from the gate lines, and source electrodes of thinfilm transistors having active layers extend from the data lines andoverlap the gate electrodes. Furthermore, drain electrodes of the thinfilm transistors define conductive channels. The drain electrodes areformed in an inverted “U” shape such the source electrodes formshielding plates for blocking incident light that is directed toward theconductive channel of the thin film transistor.

[0058] Another advantage of the present invention is to provide adriving method for driving M gate lines G₁ though G_(M). The methodincluding applying a gate signal voltage pulse to a p+1th gate line soas to form a plurality of inverted “U” shaped channels. Then,sequentially applying a gate signal voltage pulse to the pth gate lineso as to form a plurality of inverted “U” shaped channels, andsequentially applying a gate signal voltage pulse to the p−1th gate lineso as to form a plurality of inverted “U” shaped channels, wherein p+1is less than or equal to M, and wherein p−1 is greater than or equal to1.

[0059] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory and are intended to provide further explanation of theinvention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] The accompanying drawings, which are included to provide afurther understanding of the invention and are incorporated in andconstitute a part of this specification, illustrate embodiments of theinvention and together with the description serve to explain theprinciples of the invention.

[0061] In the drawings:

[0062]FIG. 1 is an exemplary view showing voltage waveforms applied to aconventional art liquid crystal display device;

[0063]FIG. 2 illustrates a unit pixel of a conventional art generaltransmission/reflective type liquid crystal display device;

[0064]FIG. 3 is an exemplary view showing the incident angle of light;

[0065]FIG. 4 is a sectional view taken along line I-I′ of FIG. 2;

[0066]FIG. 5 is a sectional view taken along line II-II″ of FIG. 2;

[0067]FIG. 6 illustrates a transmission/reflective type liquid crystaldisplay device that is in accord with the principles of the presentinvention;

[0068]FIG. 7 is an exemplary view showing a conventional method ofapplying scan signals;

[0069]FIG. 8 illustrates applied voltage waveforms of FIG. 7;

[0070]FIG. 9 is an exemplary view showing a method of applying scansignals in accordance with the principles of the present invention; and

[0071]FIG. 10 is an exemplary view showing voltage waveforms of FIG. 9.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0072] Reference will now be made in detail to the preferred embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings.

[0073]FIG. 6 is an exemplary view of a transmission/reflective typeliquid crystal display device that is in accordance with a preferredembodiment of the present invention. With reference to FIG. 6, gatelines 204 are arranged at regular intervals on a substrate, and datalines 202 are arranged at regular intervals so as to cross the gatelines 204. The gate and data lines are isolated from each other.Accordingly, the gate lines 204 and the data lines 202 form a matrix ofunit liquid crystal cell that are defined by the data lines 202 and thegate lines 204. Each liquid crystal cell includes a thin film transistor(TFT), a reflection electrode 214, and a pixel electrode 215.

[0074] Each thin film transistor (TFT) includes a gate electrode 210that extends in a predetermined direction (see below) from the gate line204, a source electrode 208 that extends from the data line 202 (as issubsequently explained) so as to overlap part of the gate electrode 210,and a drain electrode 212 that is formed on the gate electrode 210 so asto correspond with the source electrode 208. The thin film transistor(TFT) also includes an active layer (which is not shown in FIG. 6 forclarity) wherein a conductive channel is formed between the sourceelectrode 208 and the drain electrode 212 when a scan signal is appliedto the gate electrode 210 via the gate line 204. The active layer isbeneficially comprised of amorphous silicon, which can be formed at arelatively low temperature on a low-priced glass substrate.

[0075] As shown in FIG. 6, the source electrode 208 extends from thedata line 202 in an inverted hook shape, which is in contrast to theupright hook shape of the source electrode in FIG. 2. The drainelectrode 212 is isolated by a predetermined interval from the inside ofthe source electrode 208, but extends into the hook portion.Accordingly, the conductive channel formed in the active layer has aninverted “U” shape. Additionally, the gate electrode 210 extendsdownward toward the bottom of the display.

[0076] Still referring to FIG. 6, a pixel electrode 215 electricallycontacts the drain electrode 212 through a drain contact hole 216 thatis formed in an insulation film (not shown in FIG. 6) over the drainelectrode 212. The pixel electrode 215 is made of a transparentconductive material and is formed in the pixel region defined by theliquid crystal cell. Furthermore, at marginal portions of the pixelregion 215 and on the insulation film (again, which is not shown in FIG.6) is a translucent reflection electrode 214 that is comprised of areflective and conductive material. The reflection electrode 214 is alsoformed over an upper portion of the thin film transistor (TFT). Thatpart of the reflection electrode serves as a shielding plate 214A thatblocks light that is directed toward the conductive channel fromreaching the thin film transistor (TFT).

[0077] Still referring to FIG. 6, the gate line 204 extends in thedirection opposite the gate electrode 210. Part of the gate line 204acts as a first electrode of a storage capacitor 218. That storagecapacitor further includes part of the insulation film, which acts as adielectric, and a storage electrode that overlaps the first electrode.The storage electrode (which is below the pixel electrode) is connectedto the pixel electrode 215 through the storage contact hole 222.

[0078] Thus, a transmission/reflective type liquid crystal displaydevice in accord with FIG. 6 includes both a thin film transistor (TFT)that forms an inverted “U” conductive channel, and a shielding plate214A that protects that conductive channel. Accordingly, light that isincident at an angle of less than 30° (reference FIG. 3) to a displayunit of a notebook type personal computer that is in accord with theprinciples of the present invention is blocked by the source electrode208 that helps form the inverted “U”-shaped conductive channel. Thus,light that would be incident on the regions “A” to “D” of FIG. 2 isblocked. Such blocking reduces or prevents photo-induced leakage currentin the thin film transistor (TFT).

[0079] The transmission/reflective type liquid crystal display device ofthe conventional art as shown in FIG. 2 and the transmission/reflectivetype liquid crystal display device in accordance with the presentinvention as shown in FIG. 6 will now be compared. First, referring tothe conventional liquid crystal display device shown in FIG. 2, theliquid crystal cells defined by intersections of the data lines 102 andthe gate lines 104 include a thin film transistor (TFT) and the storagecapacitor 118. The thin film transistors (TFT) of the pth gate line 104include gate electrodes 110 that extend in the direction of the p−1thgate line 104. The drain electrode 112 and the source electrode 108 thatoverlap the gate electrode 110 form a “U”-shaped conductive channel.

[0080] Additionally, the storage capacitors 118 for the drains of thepth gate line 104 include a first electrode 119 formed by the p−1th gateline 104 and a storage electrode 120 that overlaps the first electrode119. Additionally, the storage electrode 120 connects to the pixelelectrode 115 through a storage contact hole 122 over the p−1th gateline.

[0081] In contrast, a transmission/reflective type liquid crystaldisplay device in accord with FIG. 6 has unit liquid crystal cellsdefined by intersections of data lines 202 and gate lines 204. Thoseunit liquid crystal cells include thin film transistors (TFT) andstorage capacitors 218. Each of those thin film transistors (TFT)includes a gate electrode 210 that extends from the pth gate line 204toward the p+1th gate line 204. Additionally, each thin film transistor(TFT) includes a drain electrode 212 and a source electrode 208 thatoverlap the gate electrode 210 so as to form an inverted “U” shapedconductive channel.

[0082] Additionally, the storage capacitors 218 for the thin filmtransistors on the pth gate line 204 each include a first electrode thatis part of the p+1th gate line 204, part of the gate insulation film,and a storage electrode that is connected to the pixel electrode 215through a storage contact hole 222 over the p+1th gate line 204. Thestorage capacitor 218 is charged to a data signal voltage during aturn-on period of the thin film transistor (TFT) when a scan signal isapplied to the pth gate line 204. The storage capacitor 218 thensupplies the charged voltage to the pixel electrode 224 during theturn-off period of the thin film transistor (TFT) to maintain the driveof the liquid crystal.

[0083] A general driving method of a conventional liquid crystal displaydevice will now be described in detail with reference to FIGS. 7 and 8.First, a common electrode voltage (Vcom) is applied to the commonelectrode. Then scan signals (V_(G1) to V_(Gm)) are sequentiallysupplied to the first gate lines (G₁) through the mth gate line (G_(m))by a gate driver integrated circuit 300. The scan signals (V_(G1) toV_(Gm)) are applied as pulses having a low potential of −5V and a highpotential of 20V. Accordingly, when a scan signal (V_(G1)) is applied tothe first gate line (G₁), the 20V high potential applied to the gateelectrodes connected to the first gate line (G₁) turn on theirassociated thin film transistors. Accordingly, conductive channels areformed. Then, the data signal voltages (V_(d)) supplied by the datadriver integrated circuit 310 to the data lines (D₁ to D_(n)) areapplied to the source electrodes of the thin film transistors connectedto the first gate line (G₁). The data signal voltages (V_(d)) passthrough the thin film transistor channels to the drain electrodes. Sincethe drain electrodes connect to the pixel electrodes through the draincontact holes, the data signal voltages (V_(d)) are applied to the pixelelectrodes. This forms electric fields to the common electrode. Thus,the liquid crystal is driven to control its light transmittance.Furthermore, since the pixel electrodes are connected to the storageelectrodes through the storage contact holes, the data signal voltages(V_(d)) applied to the pixel electrodes are charged into the storagecapacitors during turn-on. When the scan signal (V_(G1)) applied to thefirst gate line (G₁) returns to −5V, a 20V scan signal (V_(G2)) isapplied to the second gate line (G₂). Then, the thin film transistorsconnected to the first gate line (G₁) turn off, while those connected tothe second gate line turn on. The data signal voltages (V_(d)) chargedinto the storage capacitors associated with the first gate line (G₁)continue to be supplied to the pixel electrodes. This maintains thedrive to the liquid crystal cells of the first gate line (G₁). As thethin film transistors associated with the second gate line (G₂) turn on,the foregoing process is repeated for the liquid crystal cellsassociated with the second gate line.

[0084] However, with a transmission/reflective type liquid crystaldisplay device in accord with FIG. 6, since the storage capacitorsassociated with the first gate line (G₁) include part of the second gateline (G₂), the data signal voltages (V_(d)) charged into the storagecapacitors associated with the first gate line (G₁) would fluctuate ifthey were driven as in the conventional art. This is because the datasignal voltages (V_(d)) charged into the storage capacitors associatedwith the first gate line (G₁) would be impacted by the scan signalvoltage (V_(G2)) applied to the second gate line (G₂). Such fluctuationswould degrade picture quality due to flicker or an image stain.

[0085] Accordingly, to prevent such problems, a better driving methodfor a liquid crystal display device in accord with FIG. 6 will bedescribed with reference to FIGS. 9 and 10. First, a common electrodevoltage (V_(com)) is applied to the common electrode. Then, scan signals(V_(G1) to V_(Gm)) are sequentially supplied for the m gate lines (G₁through G_(m)) from a gate driver integrated circuit 400. The scansignals (V_(G1) to V_(Gm)) are applied as pulses having a low potentialof −5V and a high potential of 20V. Accordingly, when a scan signal(VG₂) is μapplied to the second gate line (G₂), a high potential isapplied to the gate electrodes of the thin film transistors associatedwith the second gate line (G₂). Conductive channels are then formedbetween the source electrodes and the drain electrodes of the associatedthin film transistors. Data signal voltages (V_(d)) are then suppliedfrom a data driver integrated circuit 410 to the data lines (D₁ toD_(n)). Those data signal voltages (V_(d)) are applied to the sourceelectrodes of the thin film transistors associated with the second gateline (G₂). Since those transistors are turned ON, the data signalvoltages (V_(d)) are applied to the drain electrodes. Since the drainelectrodes are connected to pixel electrodes via drain contact holes,the data signal voltages (V_(d)) are applied to the pixel electrodesthat are associated with the second gate line (G₂). Thus, an electricfield is applied formed with the common electrode, and thus the liquidcrystal is driven to control the light transmittance. Since the pixelelectrodes are connected to storage electrodes through storage contactholes, the data signal voltages (V_(d)) applied to the pixel electrodesare charged into the storage capacitors associated with the second gateline (G₂). When the scan signal (V_(G2)) applied to the second gate line(G₂) returns to −5V (LOW) and a scan signal (V_(G1)) applied to thefirst gate line (G₁) goes to 20V, the thin film transistors associatedwith the second gate line (G₂) turn off while the thin film transistorsassociated with the first gate line turn on.

[0086] At this time, the data signal voltage (V_(d)) charged into thestorage capacitors associated with the second gate line (G₂) arecontinuously supplied to the pixel electrode such that the drive to theliquid crystal cells associated with the second gate line (G₂) ismaintained.

[0087] In addition, as the thin film transistors associated with thefirst gate line (G₁) turn on, the liquid crystal cells, the thin filmtransistors, and the storage capacitors associated with the first gateline (G₁) are driven in the same manner as described above.

[0088] As indicated above, in the driving method of a liquid crystaldisplay devices of the present invention, unlike the general drivingmethod, the scan signals (V_(G1) to V_(Gm)) are sequentially appliedfrom the mth gate line (G_(m)) up to the first gate line (G₁). Thus, thedata signal voltages (V_(d)) charged into the storage capacitors havereduced fluctuations. When the liquid crystal display device is drivenby the driving method of the present invention, after a scan signal(V_(G2)) is applied to the second gate line (G₂) a scan signal (V_(G1))is applied to the first gate line (G₁). Thus, the data signal voltage(V_(d)) charged into the storage capacitor corresponding to the secondgate line (G₂) is not affected by the scan signal (V_(G1)) applied tothe first gate line (G₁). Accordingly, the driving method of a liquidcrystal display device in accordance with the present invention preventsdegradation of a picture quality caused by flicker or an image stain.

[0089] As described, the inventive liquid crystal display device and itsdriving method have the following advantages. Since the conductivechannels of the thin film transistors are formed in an inverted“U”-shape, the source electrodes block light incident at an incidentangle of less than 30° from reaching the conductive channels of the thinfilm transistors. Therefore, photo-induced leakage current is reduced oreliminated. This can prevent or reduce picture quality degradation. Todrive a liquid crystal display device according to the presentinvention, the scan signals are sequentially applied from the mth gateline up to the first gate line. Thus, the data signal voltage charged inthe storage capacitors are maintained, preventing pixel voltagefluctuations caused by following scan signals. This also preventspicture quality degradation such as flicker or image stains.

[0090] As the present invention may be embodied in several forms withoutdeparting from the spirit or essential characteristics thereof, itshould also be understood that the above-described embodiments are notlimited by any of the details of the foregoing description, unlessotherwise specified, but rather should be construed broadly within itsspirit and scope as defined in the appended claims, and therefore allchanges and modifications that fall within the meets and bounds of theclaims, or equivalence of such meets and bounds are therefore intendedto be embraced by the appended claims.

[0091] It will be apparent to those skilled in the art that variousmodifications and variation can be made in the present invention withoutdeparting from the spirit or scope of the invention. Thus, it isintended that the present invention cover the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A liquid crystal display device comprising: a plurality of G gate lines; a plurality of D data lines that are perpendicular to and that cross the gate lines; a gate electrode that extends from a pth gate line toward a p+1th gate line, wherein p is greater than 1 and p+1 is less than or equal to G; and a thin film transistor comprised of a semiconductor layer over the gate electrode, a source electrode that extends over the semiconductor layer from a data line, and a drain electrode, wherein an inverted “U” shaped conductive channel is formed in the semiconductor layer when a gate signal is applied to the gate electrode; and a shielding plate for blocking incident light from reaching the conductive channel; wherein the source electrode blocks incident light from reaching the conductive channel.
 2. The device of claim 1, wherein intersections of the plurality of gate lines and data lines define a plurality of liquid crystal cells, and wherein a pixel electrode is provided in each liquid crystal cell.
 3. The device of claim 1, wherein intersections of the plurality of gate lines and data lines define a plurality of liquid crystal cells, and wherein a pixel electrode and a reflective electrode are provided in each liquid crystal cell.
 4. The device of claim 2, further comprising: a pixel electrode that is electrically connected to the drain electrode through a drain contact hole in an insulation layer; and a reflective electrode on an insulation film that overlaps the pixel electrode.
 5. The device of claim 4, wherein the reflective electrode is provided at marginal portions of the liquid crystal cell.
 6. The device of claim 2, further comprising a storage capacitor that is electrically connected to the pixel electrode, wherein the storage capacitor includes a portion of the p+1th gate line.
 7. The device of claim 6, wherein the storage capacitor further includes a storage electrode that overlaps the portion of the p+1 gate line.
 8. The device of claim 1, wherein the conductive channel is made of amorphous silicon.
 9. A notebook computer, comprising: a housing comprised of a base and of a display case having an upper edge and a bottom edge; and a transmissive/reflective liquid crystal display device in the display case, the liquid crystal display device having a top that is adjacent the upper edge, wherein the liquid crystal display device includes a plurality of G gate lines, wherein the first gate line is adjacent the upper edge, and wherein the Gth gate line is adjacent the bottom edge; a plurality of data lines that are perpendicular to and cross the gate lines; a gate electrode on the pth gate line that extends toward the bottom edge; and a thin film transistor comprised of a semiconductor layer over the gate electrode, a source electrode that extends over the semiconductor layer from a data line, and a drain electrode, wherein an inverted “U” shaped conductive channel is formed in the semiconductor layer when a gate signal is applied to the gate electrode; and a shielding plate for blocking incident light from reaching the conductive channel; wherein the source electrode also blocks incident light from reaching the conductive channel.
 10. The notebook computer of claim 9, wherein intersections of the plurality of gate lines and data lines define a plurality of liquid crystal cells, and wherein a pixel electrode is provided in each liquid crystal cell.
 11. The notebook computer of claim 9, wherein intersections of the plurality of gate lines and data lines define a plurality of liquid crystal cells, and wherein a pixel electrode and a reflective electrode are provided in each liquid crystal cell.
 12. The notebook computer of claim 10, further comprising: a pixel electrode that is electrically connected to the drain electrode through a drain contact hole in an insulation layer; and a reflective electrode on an insulation film that overlaps the pixel electrode.
 13. The notebook computer of claim 11, wherein the reflective electrode is provided at marginal portions of the liquid crystal cell.
 14. The notebook computer of claim 10, further comprising a storage capacitor that is electrically connected to the pixel electrode, wherein the storage capacitor includes a portion of the p+1th gate line.
 15. The notebook computer of claim 14, wherein the storage capacitor further includes a storage electrode that overlaps the portion of the p+1 gate line.
 16. The notebook computer of claim 9, wherein the conductive channel is made of amorphous silicon.
 17. A method of driving a liquid crystal display device comprised of gate lines G₁ though G_(M), comprising: applying a gate signal voltage pulse to a p+1th gate line so as to form a plurality of inverted “U” shaped channels; and sequentially applying a gate signal voltage pulse to the pth gate line so as to form a plurality of inverted “U” shaped channels; and sequentially applying a gate signal voltage pulse to the p−1th gate line so as to form a plurality of inverted “U” shaped channels; wherein p+1 is less than or equal to M; and wherein p−1 is greater than or equal to
 1. 18. The method of driving a liquid crystal display device of claim 17, wherein each “U” shaped channel formed by applying a gate signal voltage pulse to the pth gate line charges a storage capacitor disposed over the a p+1th gate line.
 19. The method of driving a liquid crystal display device of claim 18, wherein the charge of each storage capacitor is not discharged by photo-induced leakage current. 